Explosively formed penetrator

An explosively formed penetrator (EFP), also known as an explosively formed projectile, a self-forging warhead, or a self-forging fragment, is a special type of shaped charge designed to penetrate armor effectively, from a much greater standoff range than standard shaped charges, which are more limited by standoff distance. As the name suggests, the effect of the explosive charge is to deform a metal plate into a slug or rod shape and accelerate it toward a target. They were first developed as oil well perforators by American oil companies in the 1930s, and were deployed as weapons in World War II.

A conventional shaped charge generally has a conical metal liner that is forced by an explosive blast into a hypervelocity jet of superplastic metal able to penetrate thick armor and knock out vehicles. A disadvantage of this arrangement is that the jet of metal loses effectiveness the further it travels, as it breaks up into disconnected particles that drift out of alignment.

An EFP operates on the same principle, but its liner is designed to form a distinct projectile that will maintain its shape, permitting it to penetrate armor at greater distance. The dish-shaped liner of an EFP can generate a number of distinct projectile forms, depending on the shape of the plate and how the explosive is detonated.

An EFP's penetration is more strongly affected by the density of its liner metal compared to a conventional shaped charge. At 16.654 g/cm³, tantalum is preferable in delivery systems that have limitations in size, like the SADARM, which is delivered by a howitzer. For other weapon systems without practical limitations on warhead diameter, a less expensive copper liner (8.960 g/cm³) of double the diameter can be used instead. An EFP with a tantalum liner can typically penetrate steel armor of a thickness equal to its diameter – or half that amount with a copper liner instead. By contrast, a conventional shaped charge can penetrate armor up to six times its diameter in thickness, depending on its design and liner material.

Some sophisticated EFP warheads have multiple detonators that can be fired in different arrangements causing different types of waveform in the explosive, resulting in either a long-rod penetrator, an aerodynamic slug projectile, or multiple high-velocity fragments. A less sophisticated approach for changing the formation of an EFP is the use of wire mesh in front of the liner, which causes the liner to fragment into multiple penetrators.

In addition to single-penetrator EFPs (also called single EFPs or SEFPs), there are EFP warheads whose liners are designed to produce more than one penetrator; these are known as multiple EFPs, or MEFPs. The liner of an MEFP generally comprises a number of dimples that intersect each other at sharp angles. Upon detonation, the liner fragments along these intersections to form up to dozens of small, generally spheroidal projectiles, producing an effect similar to that of a shotgun. The pattern of impacts on a target can be finely controlled based on the design of the liner and the manner in which the explosive charge is detonated. A nuclear-driven MEFP was apparently proposed by a member of the JASON group in 1966 for terminal ballistic missile defense. A related device was the proposed nuclear pulse propulsion unit for Project Orion.

Extensive research is going on in the zone between jetting charges and EFPs, which combines the advantages of both types, resulting in very long stretched-rod EFPs for short-to-medium distances (because of the lack of aerostability) with improved penetration capability.

EFPs have been adopted as warheads in a number of weapon systems, including the CBU-97 and BLU-108 air bombs (with the Skeet submunition), the M303 Special Operations Forces demolition kit, the M2/M4 Selectable Lightweight Attack Munition (SLAM), the SADARM submunition, the SMArt 155 top-attack artillery round, the Low Cost Autonomous Attack System, the TOW-2B anti-tank missile, and the NASM-SR anti-ship missile.